1. Technical Field
The present techniques generally relate to the operation and maintenance of reactors. More particularly, the present techniques relate to the reduction or inhibition of fouling of reactors, such as in polymerization reactors.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art which may be related to various aspects of the present techniques which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present techniques. Accordingly, it should be understood that these statements are to be read in this light, and not as any indication of what subject matter may constitute prior art to the present techniques.
As chemical and petrochemical technologies have advanced, the products of these technologies have become increasingly prevalent in society. In particular, as techniques for bonding simple molecular building blocks into polymers have advanced, the polymer products, typically in the form of various plastics, have been increasingly incorporated into various everyday items. For example, polyolefin polymers, such as polyethylene, polypropylene, and their copolymers, are used for retail and pharmaceutical packaging, food and beverage packaging (such as juice and soda bottles), household containers (such as pails and boxes), household items (such as appliances, furniture, carpeting, and toys), automobile components, pipes, conduits, and various industrial products.
Specific types of polyolefins, such as high-density polyethylene (HDPE), have particular applications in the manufacture of blow-molded and injection-molded goods, such as food and beverage containers, film, and plastic pipe. Other types of polyolefins, such as low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), isotactic polypropylene (iPP), and syndiotactic polypropylene (sPP) are also suited for similar applications. The mechanical requirements of the application, such as tensile strength and density, and the chemical requirements, such as thermal stability, molecular weight, and chemical reactivity, typically determine what type of polyolefin is suitable for any particular purpose.
To achieve specific performance parameters, various processes exist by which olefins may be polymerized to form polyolefins. Typically, these processes are performed at, or near, petrochemical facilities, which provide low-cost access to the short-chain olefin molecules (monomers and comonomers) such as ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 1-octene, 1-decene, and other building blocks of the much longer polyolefin polymers. These monomers and comonomers may be polymerized in a liquid-phase or gas-phase polymerization reactor to form a product including polymer (polyolefin) solid particulates, typically called fluff or granules. The fluff may possess melt, physical, rheological, and/or mechanical properties of interest, such as density, melt index (MI), melt flow rate (MFR), copolymer content, comonomer content, modulus, and crystallinity. The reaction conditions within the reactor, such as temperature, pressure, chemical concentrations, polymer production rate, and so forth, may be selected to achieve the desired fluff properties.
Reactors used for the polymerization or co-polymerization of olefins (e.g., loop reactors, liquid boiling-pool reactors, gas phase reactors, etc.) can encounter operating difficulties when the polyolefin polymer product adheres to the reactor interior wall and does not dislodge or break loose from the interior wall. This condition is known as “fouling” of the reactor. Recovery from fouling generally involves removing the adhering polymer layer, e.g., by washing the reactor with hot diluent (or a solvent), blasting the reactor interior wall with various materials (e.g., sandblasting), or by water washing the reactor with relatively high-pressure water. However, such cleaning and recovery may be expensive due to the cost associated with the maintenance (cost of cleaning), the downtime of the reactor (loss of production), and the like.
Nevertheless, removal and recovery of the polymer adhered to the reactor wall is typically necessary in olefin polymerizations and other reactions because the build-up or accumulation of polymer in the reactor and on the reactor interior wall, if left intact, may reduce heat transfer through the reactor wall or even plug the reactor. For example, fouling of the reactor wall may reduce heat transfer between the contents of the reactor and a cooling medium in a reactor jacket. As would be appreciated by one of ordinary skill in the art, polymerization of olefins (e.g., terminal olefins) is an exothermic process and heat removal is generally an important function of the reactor. In addition, reactor fouling may cause other problems, such as interfering with circulation or movement of the reactor contents. For example, large masses of fouled polymer may form or break free from the reactor wall and restrict the flow of the contents in the reactor.
Fouling can occur for a number of reasons, including reactions between materials residing on the reactor wall with the monomer, catalyst, and additive components, and so forth. Further, once even a thin layer of polymer forms on the interior surfaces of the reactor walls, there is a greater tendency for additional polymer to form on the wall. In other words, a small amount of fouling may cause more fouling by acting as a site for the adhesion of more polymer chains onto the reactor wall. In addition, a layer of polymer disposed on the reactor wall may also act as a seed for further polymerization of polymer onto the reactor wall, e.g., when active catalyst remains in the polymer material adhered to the reactor wall. Therefore, it is generally desirable to avoid adhesion of the polymer to the reactor wall as much as possible.